The findings, released Wednesday by the American Civil Liberties Union of Massachusetts, were derived from a study of more than 200,000 Field Interrogation and Observation reports made by the department that found blacks were stopped in disproportionate numbers even when accounting for area crime rates and other nonracial factors.

The A.C.L.U. requested information on the encounters in 2009 and the next year reached an agreement with the police to have the department commission the study. The researchers shared their preliminary finding with the organization and the department.

“It’s evidence of racially biased policing,” said Nusrat Choudhury, a staff attorney with the organization’s Racial Justice Program. “During the four-year period of the study, Boston police were targeting black people and black neighborhoods for police civilian encounters for reasons that aren’t explained.”

The study found that 63.3 percent of those stopped in Boston were black, and noted that blacks make up 24.4 percent of the city’s population, according to the 2010 census. It said the reason given by police officers for 75 percent of the encounters was “investigate person” — a notation that does not reference a specific crime — and that 97.5 percent of those encounters resulted in neither a documented seizure or an arrest.

A statement posted on the Boston Police Department’s website emphasized the role crime rates played in the encounters. “The study showed that the amount of crime in a neighborhood is the most powerful predictor of the number of F.I.O.’s done in a neighborhood,” the statement said.

But it added, “The study did show some racial disparities that must be addressed.”

In the statement, the department acknowledged that blacks were 8 percent more likely to be stopped repeatedly, and 12 percent more likely to be stopped and frisked when factors like criminal history and gang affiliation were taken into account.

The department said that it had already taken a number of steps since 2010 to remedy the disparity, including increased training on racial bias and profiling and improved record keeping, and that it planned more training, reporting and accountability measures going forward.

In a statement, Mayor Martin J. Walsh noted that the years studied preceded his administration, but said that “we’ve acknowledged and worked to address the racial disparities outlined by the A.C.L.U.” and said he had appointed the department’s most diverse command staff to date.

The report comes as police actions disproportionately affecting blacks have made headlines in other cities, like New York and Newark, and amid broad attention toward profiling after an unarmed black teenager was shot and killed by a white police officer in Ferguson, Mo.

“The numbers are confirming around the country, sometimes to different degrees, that people of color are being targeted not because they’re doing anything wrong, but because of who they are and how they look,” Ms. Choudhury said.